1. Field of Invention
This invention relates to gasification of high ash bituminous coals that have high ash fusion temperatures. Existing fluidized bed gasifiers are unsuitable to process such coals economically as these coals are less reactive which leads to lower carbon conversion and generating undesirable components such as tar. If such coals are gasified in slagging entrained flow gasifiers that operate at higher temperatures to improve carbon conversion, the large energy penalty associated with slags, containing a large amount of additives that are necessary to lower ash fusion temperature, make the process economically unviable. In this invention, such coals are dealt with a two stage gasification process—a primary gasification step followed by a high temperature partial oxidation step of residual char carbon and small quantities of tar. The process is further beneficial with the inclusion of an internally circulating fluidized bed to effectively cool the high temperature syngas.
2. Background and Related Art
Those of skill in the art of coal gasification know that some bituminous coals are unsuitable for use in existing commercial gasifiers economically or practically. The initial ash deformation temperatures of these bituminous coals as measured by ASTM D-1857 are well above 1500° C. It becomes very difficult to melt the ash for gasifiers that rely on slagging the ash in the gasification process, such as conventional GE, Shell and E-Gas gasifiers. For these and other such gasifiers, to gasify the high ash fusion temperature coals, the gasifier operating temperature will be too high even with added fluxing agents and such operation shortens the life span of linings in the gasifier. Further, the high ash bituminous coals can contain up to approximately 45 weight percent (wt %) ash in the coal. Even with addition of, for example, approximately 20 wt % fluxing agents to lower the coal ash fusion temperature, the energy penalty to melt the large amount of ash is simply too high and leads to an inefficient and unreliable gasification process. Further, it would be difficult to operate these gasifiers due to large amount of slag flow of combined ash and fluxing agent. The high ash and high ash fusion temperature bituminous coals have been precluded from many existing gasification technologies.
It is also difficult to gasify these coals in conventional fluidized bed gasifiers as the bituminous coals have quite low reactivities with gasification agents. The fundamental reason for the low reactivity in a fluidized bed is that the operating temperature is limited due to the tendency for clinker formation. Once clinkers form, the gasifier looses fluidization and functional capabilities. Although the ash fusion temperature is high, the gasifier will form clinker a few hundred degrees of Celsius below the ash fusion temperature as the surfaces of burning coal particles have a much higher temperature than the measured bulk temperature in the fluidized bed. Further, the temperature in a fluidized bed gasifier is rarely uniform due to hot spots in some parts of the bed that tend to melt the surface of coal ash particles, leading to agglomerates and eventual clinker formation. Therefore, it is very rare for a fluidized bed gasifier to operate above approximately 1100° C. without bed fouling in spite of the coal ash fusion temperatures well above approximately 1500° C. Because of the operating temperature limitations, the carbon conversion in the fluidized bed process is generally below approximately 90%. The remaining carbon has to be combusted in a combustor (with all of the associated equipment in the combustion train) for economic viability, leading to increased capital and operating and maintenance costs for the gasification process. Thus, existing fluidized bed gasifiers cannot handle bituminous coals economically. Further, gasification of bituminous coals in fluidized beds generates small quantities of tar in the syngas, which is hard to remove and it becomes expensive to treat the syngas. Without treatment for tar in the syngas, the downstream equipment such as syngas cooler and dust filters tend to foul, leading to operational reliability concerns.
It is more difficult to gasify these types of bituminous coals in a moving bed gasifier. Most bituminous coals have some caking tendency and the moving bed gasifier has difficulty handling caking coals. The carbon conversion is even lower than in fluidized bed gasifiers due to limitations related to operating temperature. In addition, the moving bed gasifier generates large amount of tar and phenol water that requires expensive processes to treat to meet today's environmental regulations.
Two-stage gasification is known. The fixed bed or moving bed two-stage gasifier was developed to produce two different syngas streams in U.S. Pat. No. 5,139,535. One stream contains tar and carbonization gas and the other is product syngas from coal gasification. Due to low capacity, lower yield of product syngas and high wastewater production, the two stage moving bed gasifiers are obsolete.
There are various two-stage fluidized bed gasification systems. One type uses a two-vessel arrangement with a combustor and a gasifier. The flue gas from the combustor together with hot solids recycling between the gasifier and the combustor is fed into the gasifier to provide heat for the endothermic gasification reactions. U.S. Pat. No. 4,386,940 discloses one of these types. However, those skilled in the art of gasification understand that the problem is not how to provide the heat to the gasifier, but how to convert enough carbon and coal into desirable syngas constituents carbon monoxide and hydrogen. In the normal operating temperature range of up to approximately 1100° C. in such a two stage system, the coal conversion to carbon monoxide and hydrogen is too low with undesirable components such as tar still present in the syngas. Therefore, combustion and gasification in two separate vessels, and then routing the flue gas to the gasifier, is essentially no different than using a single gasifier with combustion and gasification zones.
U.S. Patent Publication No. 2013-0056685 discloses using a two-stage gasifier to accomplish high carbon conversion. The first-stage gasifier or pyrolyzer operates at approximately 500-700° C. and the second-stage operates at 1400-1500° C. The ash from the second-stage gasifier is melted and discharged as molten slag. This concept is similar to the one of U.S. Pat. No. 6,455,011 that discloses a method to gasify waste in a two-stage gasifier system. The first-stage gasifier is a fluidized bed gasifier and the second-stage is a swirl or cyclonic gasifier and ash is melted and discharged as slag. Yet, these methods incorporate the same difficulties and poor economics in handling high ash bituminous coals with high ash fusion temperatures as the entrained flow gasifiers.
Another two-stage entrained flow slagging gasifier is disclosed in U.S. Pat. No. 8,444,724. Since this type of gasifier requires melting and slagging the ash and fluxing agents, it cannot viably be used for those coals with high ash content and high ash fusion temperatures.
It is thus readily apparent that present coal gasification technologies cannot economically process coals with high ash content and high ash fusion temperatures. In addition to ably gasifying such coals, the layout of the process and design of downstream equipment also plays a significant role in skillfully generating high yields of nearly dust-free syngas for chemical synthesis or power generation end use.
It is an intention of the present invention to provide a process, appropriate apparatus and method for operating the series of apparatus that can gasify the high ash, high ash fusion temperature bituminous coals with carbon conversions above approximately 90%, and preferably above approximately 98%, while providing nearly tar-free syngas for further processing downstream to end-use chemicals or power generation.